The present invention relates, in general, to an angiostatin receptor and, in particular, to an angiostatin receptor present on cellular plasma membranes. More particularly, the present invention relates to the human angiostatin receptor, ATP synthase, or subunit or portion thereof, and to the use thereof in assays designed to screen compounds for their ability to serve as agonists or antagonists of human angiostatin. The invention further relates to nucleic acid sequences encoding ATP synthase, or subunit or portion thereof, and to host cells transformed therewith. The invention also relates to antibodies specific for ATP synthase.
Tumor growth requires persistent blood vessel generation in the process of angiogenesis. If vascularization is prevented, tumor growth is dramatically impaired and the tumor size is restricted. Modulation of endogenous angiogenic inhibitors thus plays an important role in tumor development. Angiostatin, a proteolytic fragment of plasminogen, is a potent inhibitor of angiogenesis and the growth of tumor cell metastases (O""Reilly et al, Cell 79:315-328 (1994)). Angiostatin can be generated in vitro by limited proteolysis of plasminogen (Sottrup-Jensen et al, Progress in Chemical Fibrinolysis and Thrombolysis 3:191-209 (1978)) resulting in a 38 kDa plasminogen fragment (Val79-Pro353). Although the enzymatic mechanism by which angiostatin is generated in vivo is unknown, recent studies have demonstrated that the cleavage of plasminogen to angiostatin can be catalyzed by a serine proteinase (Gately et al, Cancer Research 36:4887-4890 (1996)) and a macrophage metalloelastase (Dong et al, Cell 88:801-810 (1997)). Generation of angiostatin from reduction of plasmin has also been shown in vitro (Gately et al, PNAS 94:10868-10872 (1997)) and in Chinese hamster ovary and human fibrosarcoma cells (Stathakis et al, JBC 272(33):20641-20645 (1997)).
Cellular receptors for plasminogen have previously been demonstrated on human umbilical vein endothelial cells (HUVEC) and are believed to function in the regulation of endothelial cell activities, including angiogenesis (Hajjar et al, J. Biol. Chem. 261(25):11656-11662 (1986), Hajjar et al, JBC 269(3):21191-21197 (1994)). Receptors for plasminogen are also expressed in high numbers on tumor cells, where they have been identified as critical for tumor invasion. Proteins normally found in the cytoplasm have also been shown on cell surface membranes and serve as plasminogen binding sites (Miles et al, Biochemistry 30:1682-1691 (1991)).
The present invention results from the demonstrations that plasminogen and angiostatin bind to distinct sites on cellular plasma membranes and that ATP synthase is the angiostatin binding protein. These findings make possible assays that can be used to screen compounds for their ability to modulate angiostatin activities. Compounds so identified have profound usefulness as therapeutic agents.
The present invention relates to an angiostatin receptor present on cellular plasma membranes. More particularly, the present invention relates to the human angiostatin receptor, ATP synthase, and to the use thereof, or of subunits or portions thereof, in assays designed to screen compounds for their ability to modulate angiostatin activities. The invention further relates to a nucleic acid sequences encoding ATP synthase, or subunit or portion thereof, and to host cells transformed therewith. The invention also relates to antibodies specific for ATP synthase.